1. Field of the Invention
The present invention relates to oxidation-stable iron oxide pigments which have a coating of at least one organic substance, to their preparation and to their use.
Iron oxide pigments containing iron in the +2 oxidation state constitute thermodynamically unstable phases in comparison to iron(III) oxide, Fe2O3. In the presence of air or oxygen they may undergo full or partial oxidation, for example2Fe3O4+½O2→3Fe2O3
Such reactions are known, for example, in the case of black iron oxide pigments which correspond in terms of their composition and structure to magnetite. As a result of the oxidation the pigment suffers loss in its most important quality, the colour, and hence becomes unusable. The propensity to undergo oxidation increases in line with the degree of fine division and thus with the specific surface area of the pigments.
The same applies to mixtures of black iron oxide with other iron oxide colour pigments—red iron oxide and/or yellow iron oxide—as are produced for brown shades.
If with the colour pigments it is the loss of colouristic properties that makes them unusable, then with the iron(II)-containing magnetic pigments it is the loss of the magnetic properties, which is induced likewise as a result of oxidation. Particularly at risk are finely divided magnetite pigments and also mixed phases between magnetite (Fe3O4) and maghemite (γ-Fe2O3) with a high iron(II) content. Also sensitive to oxidation, however, are mixed phases of magnetite and/or ferrites, such as cobalt ferrite, for example, and those magnetic pigments which are composed of a core of Fe3O4 and γ-Fe2O3 that is surrounded by a shell of magnetic metal oxides—particularly of iron and of cobalt. In the technical literature one finds, besides the mixed phase designation, the expression “berthollides” for the compositions described here.
2. Brief Description of the Prior Art
DE 27 44 598 A1 discloses reducing the oxidation sensitivity of finely divided ferrimagnetic magnetite particles by treatment with heterocyclic organic compounds. This treatment provides a considerable improvement over the untreated pigments, but one which cannot be raised above a certain level. Morpholine, N-(3-aminopropyl)morpholine, N-(2-hydroxyethyl)piperazine, 1,2,4-triazole and 3-amino-1,2,4-triazole have proved to be particularly effective. The heterocycles used are merely physisorbed on the pigment and are therefore among the water-soluble constituents. This may result in incompatibilities in different binder systems. The magnetite particles are treated with the heterocyclic organic compound, moreover, under a non-oxidizing atmosphere, and the presence of air must be ruled out. The process is therefore costly and complicated.
DE 41 39 052 A1 discloses iron oxide pigments which in order to increase the stability towards atmospheric oxidation are coated with cyclic carboxylic hydrazides of the general formula

Preference is given to using “phthalic hydrazide” and “maleic hydrazide”. When looked at more closely, however, all of the compounds of the general formula given above are merely 3,6-dihydroxy-substituted derivatives of pyridazine, since for the compounds of the general formula given above the following tautomeric form exists:

For “phthalic hydrazide” and “maleic hydrazide”, therefore, the names 1,4-dihydroxyphthalazine and 3,6-dihydroxypyridazine, respectively, are also customary. Thus the cyclic carboxylic hydrazides disclosed in DE 41 39 052 A1 are simple nitrogen-containing heterocycles. As already mentioned above, these are merely physisorbed on the pigment and are therefore included among the water-soluble constituents. This may result in incompatibilities in different binder systems.
DE 37 26 048 A1 describes a coating of iron oxide pigments with derivatives of benzoic acid conforming to the general formula
where    X=O or NH and    R=an optionally substituted linear or branched alkyl or alkylene radical having 1 to 30 carbon atoms or an optionally substituted cycloalkyl radical having 5 to 12 carbon atoms or an optionally substituted aryl radical having 5 to 10 carbon atoms or hydrogen. Particular preference is given to salicylic esters containing aliphatic alcohol components having 2 to 18 carbon atoms, such as octyl salicylate or dodecyl salicylate, for example. The derivatives of benzoic acid of the general formula that are used are merely physisorbed on the pigment and are therefore included among the water-soluble constituents. This may result in incompatibilities in different binder systems.
The prior art also describes numerous inorganic coatings, and coatings of mixtures of organic and inorganic substances, for the purpose of enhancing the oxidation stability of iron oxide pigments.
DE 32 11 327 A1, for instance, describes aftertreatment with boron-containing compounds. Those contemplated include ortho-boric acid, tetra-boric acid, meta-boric acid, glassy diboron trioxide, crystalline diboron trioxide, trimethyl borate, triethyl borate, complexes of boric acid with polyhydroxy compounds, and salts of boric acids, such as NH4B5O8*4H2O, Na2B4O6*10H2O, CaB3O5OH*2H2O or NaBO2, for example. As well, boron/nitrogen compounds or boron-sulphur compounds are mentioned. Preference as boron compounds is given to using boron oxides, boric acids and/or salts of boric acids, with particular preference ortho-boric acid and/or diboron trioxide. In the examples disclosed, pulverized ortho-boric acid and pulverized diboron trioxide are added. The oxidation stability of the pigments stabilized with boric acid is comparable with that of the pigments coated with heterocyclic compounds, with a reduction in the water-soluble fractions at the same time.
DE 43 22 886 A1 discloses iron oxide pigments which have been coated with oxides or hydroxides of boron, of aluminium and/or of silicon, and also with aromatic carboxylic acids of the general formulaAr—(COOX)nwhere Ar is an aromatic optionally substituted by halogen, NH2, OH, NHR, NR2, OR or R, in which R is a linear or branched alkyl radical having 1 to 30 carbon atoms or an optionally substituted aryl radical having 6 to 10 carbon atoms, and X is hydrogen, alkali metal, NR14 with R1=H, alkyl and/or aryl, ½ alkaline earth metal, ⅓ Al or ⅓ Fe, and n is an integer from 1 to 10. In the examples disclosed, mixtures of boric acid and benzoic acid or of boric acid and 4-hydroxybenzoic acid are added.
A committee of experts of the European Union has proposed classing boric acid, borates of ortho-boric acid, such as Na3BO3, and of tetra-boric acid, such as Na2B4O6*10H2O or Na2B4O6*5H2O, for example, and also diboron trioxide, as toxic to reproduction in category 2 (substances which on the basis of clear findings from animal experimentation or of other relevant information should be regarded as embryotoxic or as impairing fertility in humans) and as “toxic” (hazard symbol T) (see the Summary Record of a consultation in connection with the EU “Commission Working Group of Specialised Experts in the Field of Reprotoxicity”, Ispra, 5-6 Oct. 2004—date of Record: 22 Nov. 2004). On 8 Sep. 2005 the EU Technical Committee “Classification and Labelling” dealt conclusively with the compounds specified above. By a majority decision the recommendation of the committee of experts was followed, and for these classes of substance the classification “toxic to reproduction category 2” recorded. The R phrases that apply are 60 (May impair fertility) and 61 (May cause harm to the unborn child), and also the hazard symbol T (“toxic”) applies (see Summary Record of the meeting of EU Technical Committee “Classification and Labelling”, Arona, 8 Sep. 2005). Handling these substances, or using them in a production operation for stabilizing the iron(II)-containing iron oxide pigments, therefore requires corresponding measures in order to protect employees and the environment. For this reason the replacement of the abovementioned compounds is desirable.
It was an object of the present invention, therefore, to provide oxidation-stable iron(II)-containing iron oxide pigments whose oxidation stability is at least as good as that of boron-containing iron oxide pigments but which can be prepared in one step, without the use of inorganic substances, using commercially customary organic substances.